1. Field of the Invention
This invention relates to improvements in a cable having an optical fiber core wire or thread or a conductor core wire. More particularly, the invention relates to a water penetration preventive cable such that so-called "running of water" is prevented, namely water which has come into the cable upon damages of the water covering is prevented from running longitudinally of the cable and particularly to the improvements in the cable.
A cable having optical fiber core wires or a cable having conductor core wires is known, and the former is particularly noted for expected future prosperity upon development of an optical communication technique.
Such cables have various structures in view of the type of the assembly of core wires or threads, such as layer-strand type, ribbon type, slot ribbon type, slot type or loose pipe type. They generally have a sheath or covering to protect the core wires therein against an external atmosphere. However, the damage of the sheath due to an ageing deterioration or a damage accident is unavoidable. One of the defects when the sheath is damaged is adverse influence to the core wire due to the penetration of the water. If water once penetrates in the cable, it runs longitudinally of the cable to increase the damage, which is inherent in the structure of the cable.
2. Related Art
Such damage due to the penetration of water in the cable is already recognized, and several prior inventions of so-called "water penetration preventive cable" have been heretofore proposed. They are all devised to fill up a gap or a vacant space in the cable which will make a passage of penetrated water. One such method comprises a step of concentrically stranding core wires while coating the core wires with water repellent jelly (more particularly, a mixture of petrolatum or polybutene as a base, with petroleum wax, low molecular weight polyethylene, and an antioxidation agent), or in case of strand type cable or loose pipe type cable, the method comprises coating the pipe with the jelly. Since this method can sufficiently fill the jelly in the space between the core wires, it has large water penetration preventing effect. However, this method scarcely avoids the deterioration of working environment due to the drop of the jelly in the step of assembling the core wires.
In order to solve the problem of filling a vacancy in a cable with a jelly in situ for the stranded wires, there is also known a method of pressuring the above-described jelly in a vacancy within a cable core or a pipe containing a plurality of concentrically stranded core wires when a layer strand cable or loose pipe type cable is concerned. Further, there is known a method of pressuring a water repellent jelly into a vacancy in a cable which, in the case of a ribbon type cable, is a space between the laminate of the ribbon and a sheath or which, in the case of a slot ribbon type cable, is a space except the laminate in the slot of a core material or which, in the case of a slot type cable, is a space of the slot of the core material.
However, operability can be improved, but when the array of the core wires is dense, it is difficult to uniformly fill the vacancy within the cable with the jelly, and unfilled spaces tend to occur. In view of this point, the jelly is pressured in a heated state so as to increase the fluidity of the jelly, but after the jelly is pressured-in, void can be generated due to the thermal shrinkage of the jelly to form a water passage. On the other hand, if the pressuring-in is conducted under much increased pressure a problem could occur in case where the core wires are optical fibers, namely the characteristics of the fiber might be reduced.
In order to solve problems unavoidable for the method of pressuring-in of a water repellent jelly, it is proposed to add high water absorptive resin to the jelly. This method is advantageous in the improvement of water penetration preventive effect, but since hydrophilic, high water absorptive resin is dispersed in the oily jelly, oil content disturbs the water absorption so that the capacity of the high water absorptive resin is not sufficiently performed, thereby not completely solving it. In order to prevent the capacity of the high water absorptive resin from decreasing, there is also proposed a method of stranding core wires after coating the core wires with silicone oil and then putting a powder-like high water absorptive resin adhering to the wires. In this method, it is difficult to regulate the adhering amount of the high water absorptive resin powder, and working environment is deteriorated due to the scatter of the resin powder.
On the other hand, there is known a method of preventing water from penetrating to a cable by taping the outside of the core wire with a sheet which has a particulate form of the high water absorptive resin fixed with a binder to a base such as film or nonwoven fabric, which resin will swell upon contact with water which may have come into the cable. However, this method has some problems. That is, since the water absorptive resin is, in general, not fixed firmly to the base material, the resin tends to be frequently dropped at the time of manufacturing a cable, and, further, while a thin sheet is required for the core wire of small diameter, such a thin sheet is not easily manufactured.